Internet Engineering Task Force (IETF) I. Gashinsky
Request for Comments: 6583 Yahoo!
Category: Informational J. Jaeggli
ISSN: 2070-1721 Zynga
W. Kumari
Google, Inc.
March 2012
Operational Neighbor Discovery Problems
Abstract
In IPv4, subnets are generally small, made just large enough to cover
the actual number of machines on the subnet. In contrast, the
default IPv6 subnet size is a /64, a number so large it covers
trillions of addresses, the overwhelming number of which will be
unassigned. Consequently, simplistic implementations of Neighbor
Discovery (ND) can be vulnerable to deliberate or accidental denial
of service (DoS), whereby they attempt to perform address resolution
for large numbers of unassigned addresses. Such denial-of-service
attacks can be launched intentionally (by an attacker) or result from
legitimate operational tools or accident conditions. As a result of
these vulnerabilities, new devices may not be able to "join" a
network, it may be impossible to establish new IPv6 flows, and
existing IPv6 transported flows may be interrupted.
This document describes the potential for DoS in detail and suggests
possible implementation improvements as well as operational
mitigation techniques that can, in some cases, be used to protect
against or at least alleviate the impact of such attacks.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6583.
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RFC 6583 Operational ND Problems March 20121. Introduction
This document describes implementation issues with IPv6's Neighbor
Discovery protocol that can result in vulnerabilities when a network
is scanned, either by an intruder or through the use of scanning
tools that perform network inventory, security audits, etc. (e.g.,
"nmap").
This document describes the problem in detail, suggests possible
implementation improvements, as well as operational mitigation
techniques, that can, in some cases, protect against such attacks.
The RFCs generally describe the behavior of protocols, that is,
"what" is to be done by a protocol, but not exactly "how" it is to be
implemented. The exact details of how best to implement a protocol
will depend on the overall hardware and software architecture of a
particular device. The actual "how" decisions are (correctly) left
in the hands of implementors, so long as implementation differences
will generally produce proper on-the-wire behavior.
While reading this document, it is important to keep in mind that
discussions of how things have been implemented beyond basic
compliance with the specification is not within the scope of the
Neighbor Discovery RFCs.
1.1. Applicability
This document is primarily intended for operators of IPV6 networks
and implementors of [RFC4861]. The document provides some
operational considerations as well as recommendations to increase the
resilience of the Neighbor Discovery protocol.
2. The Problem
In IPv4, subnets are generally small, made just large enough to cover
the actual number of machines on the subnet. For example, an IPv4
/20 contains only 4096 address. In contrast, the default IPv6 subnet
size is a /64, a number so large it covers literally billions of
billions of addresses, the overwhelming majority of which will be
unassigned. Consequently, simplistic implementations of Neighbor
Discovery may fail to perform as desired when they perform address
resolution of large numbers of unassigned addresses. Such failures
can be triggered either intentionally by an attacker launching a
denial-of-service attack (DoS) [RFC4732] to exploit this
vulnerability or unintentionally due to the use of legitimate
operational tools that scan networks for inventory and other
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RFC 6583 Operational ND Problems March 2012
purposes. As a result of these failures, new devices may not be able
to "join" a network, it may be impossible to establish new IPv6
flows, and existing IPv6 transport flows may be interrupted.
Network scans attempt to find and probe devices on a network.
Typically, scans are performed on a range of target addresses, or all
the addresses on a particular subnet. When such probes are directed
via a router, and the target addresses are on a directly attached
network, the router will attempt to perform address resolution on a
large number of destinations (i.e., some fraction of the 2^64
addresses on the subnet). The router's process of testing for the
(non)existence of neighbors can induce a denial-of-service condition,
where the number of necessary Neighbor Discovery requests overwhelms
the implementation's capacity to process them, exhausts available
memory and replaces existing in-use mappings with incomplete entries
that will never be completed. A directed DoS attack may seek to
intentionally create similar conditions to those created
unintentionally by a network scan. The resulting network disruption
may impact existing traffic, and devices that join the network may
find that address resolution attempts fail. The DoS as a consequence
of network scanning was previously described in [RFC5157].
In order to mitigate risk associated with this DoS threat, some
router implementations have taken steps to rate-limit the processing
rate of Neighbor Solicitations (NS). While these mitigations do
help, they do not fully address the issue and may introduce their own
set of issues to the Neighbor Discovery process.
3. Terminology
Address Resolution: Address resolution is the process through which
a node determines the link-layer address of a neighbor given only
its IP address. In IPv6, address resolution is performed as part
of Neighbor Discovery [RFC4861], Section 7.2.
Forwarding Plane: The part of a router responsible for forwarding
packets. In higher-end routers, the forwarding plane is typically
implemented in specialized hardware optimized for performance.
Steps in the forwarding process include determining the correct
outgoing interface for a packet, decrementing its Time To Live
(TTL), verifying and updating the checksum, placing the correct
link-layer header on the packet, and forwarding it.
Control Plane: The part of the router implementation that maintains
the data structures that determine where packets should be
forwarded. The control plane is typically implemented as a
"slower" software process running on a general purpose processor
and is responsible for such functions as communicating network
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RFC 6583 Operational ND Problems March 2012
status changes via routing protocols, maintaining the forwarding
table, performing management, and resolving the correct link-layer
address for adjacent neighbors. The control plane "controls" the
forwarding plane by programming it with the information needed for
packet forwarding.
Neighbor Cache: As described in [RFC4861], the data structure that
holds the cache of (amongst other things) IP address to link-layer
address mappings for connected nodes. As the information in the
Neighbor Cache is needed by the forwarding plane every time it
forwards a packet, it is usually implemented in an Application-
specific Integrated Circuit (ASIC).
Neighbor Discovery Process: The Neighbor Discovery Process (NDP) is
that part of the control plane that implements the Neighbor
Discovery protocol. NDP is responsible for performing address
resolution and maintaining the Neighbor Cache. When forwarding
packets, the forwarding plane accesses entries within the Neighbor
Cache. When the forwarding plane processes a packet for which the
corresponding Neighbor Cache Entry (NCE) is missing or incomplete,
it notifies NDP to take appropriate action (typically via a shared
queue). NDP picks up requests from the shared queue and performs
any necessary discovery action. In many implementations, the NDP
is also responsible for responding to router solicitation
messages, Neighbor Unreachability Detection (NUD), etc.
4. Background
Modern router architectures separate the forwarding of packets
(forwarding plane) from the decisions needed to decide where the
packets should go (control plane). In order to deal with the high
number of packets per second, the forwarding plane is generally
implemented in hardware and is highly optimized for the task of
forwarding packets. In contrast, the NDP control plane is mostly
implemented in software processes running on a general purpose
processor.
When a router needs to forward an IP packet, the forwarding plane
logic performs the longest match lookup to determine where to send
the packet and what outgoing interface to use. To deliver the packet
to an adjacent node, the forwarding plane encapsulates the packet in
a link-layer frame (which contains a header with the link-layer
destination address). The forwarding plane logic checks the Neighbor
Cache to see if it already has a suitable link-layer destination, and
if not, places the request for the required information into a queue,
and signals the control plane (i.e., NDP) that it needs the link-
layer address resolved.
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RFC 6583 Operational ND Problems March 2012
In order to protect NDP specifically and the control plane generally
from being overwhelmed with these requests, appropriate steps must be
taken. For example, the size and fill rate of the queue might be
limited. NDP running in the control plane of the router dequeues
requests and performs the address resolution function (by performing
a neighbor solicitation and listening for a neighbor advertisement).
This process is usually also responsible for other activities needed
to maintain link-layer information, such as Neighbor Unreachability
Detection (NUD).
By sending appropriate packets to addresses on a given subnet, an
attacker can cause the router to queue attempts to resolve so many
addresses that it crowds out attempts to resolve "legitimate"
addresses (and in many cases becomes unable to perform maintenance of
existing entries in the Neighbor Cache, and unable to answer Neighbor
Solicitation). This condition can result in the inability to resolve
new neighbors and loss of reachability to neighbors with existing
NCEs. During testing, it was concluded that four simultaneous nmap
sessions from a low-end computer were sufficient to make a router's
Neighbor Discovery process unusable; therefore, forwarding became
unavailable to the destination subnets.
The failure to maintain proper NDP behavior whilst under attack has
been observed across multiple platforms and implementations,
including the largest modern router platforms available (at the
inception of work on this document).
5. Neighbor Discovery Overview
When a packet arrives at (or is generated by) a router for a
destination on an attached link, the router needs to determine the
correct link-layer address to use in the destination field of the
Layer 2 encapsulation. The router checks the Neighbor Cache for an
existing Neighbor Cache Entry for the neighbor, and if none exists,
invokes the address resolution portions of the IPv6 Neighbor
Discovery [RFC4861] protocol to determine the link-layer address of
the neighbor.
[RFC4861], Section 5.2, outlines how this process works. A very
high-level summary is that the device creates a new Neighbor Cache
Entry for the neighbor, sets the state to INCOMPLETE, queues the
packet, and initiates the actual address resolution process. The
device then sends out one or more Neighbor Solicitations, and when it
receives a corresponding Neighbor Advertisement, completes the
Neighbor Cache Entry and sends the queued packet.
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RFC 6583 Operational ND Problems March 20126. Operational Mitigation Options
This section provides some feasible mitigation options that can be
employed today by network operators in order to protect network
availability while vendors implement more effective protection
measures. It can be stated that some of these options are "kludges",
and can be operationally difficult to manage. They are presented, as
they represent options we currently have. It is each operator's
responsibility to evaluate and understand the impact of changes to
their network due to these measures.
6.1. Filtering of Unused Address Space
The DoS condition is induced by making a router try to resolve
addresses on the subnet at a high rate. By carefully addressing
machines into a small portion of a subnet (such as the lowest
numbered addresses), it is possible to filter access to addresses not
in that assigned portion of address space using Access Control Lists
(ACLs), or by null routing, features which are available on most
existing platforms. This will prevent the attacker from making the
router attempt to resolve unused addresses. For example, if there
are only 50 hosts connected to an interface, you may be able to
filter any address above the first 64 addresses of that subnet by
null-routing the subnet carrying a more specific /122 route or by
applying ACLs on the WAN link to prevent the attack traffic reaching
the vulnerable device.
As mentioned at the beginning of this section, it is fully understood
that this is ugly (and difficult to manage); but failing other
options, it may be a useful technique especially when responding to
an attack.
This solution requires that the hosts be statically or statefully
addressed (as is often done in a datacenter), and they may not
interact well with networks using [RFC4862].
6.2. Minimal Subnet Sizing
By sizing subnets to reflect the number of addresses actually in use,
the problem can be avoided. For example, [RFC6164] recommends sizing
the subnets for inter-router links so they only have two addresses (a
/127). It is worth noting that this practice is common in IPv4
networks, in part to protect against the harmful effects of Address
Resolution Protocol (ARP) request flooding.
Subnet prefixes longer than a /64 are not able to use stateless auto-
configuration [RFC4862], so this approach is not suitable for use
with hosts that are not statically configured.
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RFC 6583 Operational ND Problems March 20126.3. Routing Mitigation
One very effective technique is to route the subnet to a discard
interface (most modern router platforms can discard traffic in
hardware / the forwarding plane) and then have individual hosts
announce routes for their IP addresses into the network (or use some
method to inject much more specific addresses into the local routing
domain). For example, the network 2001:db8:1:2:3::/64 could be
routed to a discard interface on "border" routers, and then
individual hosts could announce 2001:db8:1:2:3::10/128, 2001:db8:1:2:
3::66/128 into the IGP. This is typically done by having the IP
address bound to a virtual interface on the host (for example, the
loopback interface), enabling IP forwarding on the host and having it
run a routing daemon. For obvious reasons, host participation in the
IGP makes many operators uncomfortable, but it can be a very powerful
technique if used in a disciplined and controlled manner. One method
to help address these concerns is to have the hosts participate in a
different IGP (or difference instance of the same IGP) and carefully
redistribute into the main IGP.
6.4. Tuning of the NDP Queue Rate Limit
Many implementations provide a means to control the rate of
resolution of unknown addresses. By tuning this rate, it may be
possible to ameliorate the issue, as with most tuning knobs
(especially those that deal with rate-limiting), the attack may be
completed more quickly due to the lower threshold. By excessively
lowering this rate, you may negatively impact how long the device
takes to learn new addresses under normal conditions (for example,
after clearing the Neighbor Cache or when the router first boots).
Under attack conditions, you may be unable to resolve "legitimate"
addresses sooner than if you had just left the parameter untouched.
It is worth noting that this technique is worth investigating only if
the device has separate queues for resolution of unknown addresses
and the maintenance of existing entries.
7. Recommendations for Implementors
This section provides some recommendations to implementors of IPv6
Neighbor Discovery.
At a high-level, implementors should program defensively. That is,
they should assume that attackers will attempt to exploit
implementation weaknesses, and they should ensure that
implementations are robust to various attacks. In the case of
Neighbor Discovery, the following general considerations apply:
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RFC 6583 Operational ND Problems March 2012
Manage Resources Explicitly: Resources such as processor cycles,
memory, etc., are never infinite, yet with IPv6's large subnets,
it is easy to cause NDP to generate large numbers of address
resolution requests for nonexistent destinations. Implementations
need to limit resources devoted to processing Neighbor Discovery
requests in a thoughtful manner.
Prioritize: Some NDP requests are more important than others. For
example, when resources are limited, responding to Neighbor
Solicitations for one's own address is more important than
initiating address resolution requests that create new entries.
Likewise, performing Neighbor Unreachability Detection, which by
definition is only invoked on destinations that are actively being
used, is more important than creating new entries for possibly
nonexistent neighbors.
7.1. Prioritize NDP Activities
Not all Neighbor Discovery activities are equally important.
Specifically, requests to perform large numbers of address
resolutions on non-existent Neighbor Cache Entries should not come at
the expense of servicing requests related to keeping existing, in-use
entries properly up to date. Thus, implementations should divide
work activities into categories having different priorities. The
following gives examples of different activities and their importance
in rough priority order. If implemented, the operation and priority
of these should be configurable by the operator.
1. It is critical to respond to Neighbor Solicitations for one's own
address, especially for a router. Whether for address resolution
or Neighbor Unreachability Detection, failure to respond to
Neighbor Solicitations results in immediate problems. Failure to
respond to NS requests that are part of NUD can cause neighbors
to delete the NCE for that address and will result in follow-up
NS messages using multicast. Once an entry has been flushed,
existing traffic for destinations using that entry can no longer
be forwarded until address resolution completes successfully. In
other words, not responding to NS messages further increases the
NDP load and causes ongoing communication to fail.
2. It is critical to revalidate one's own existing NCEs in need of
refresh. As part of NUD, ND is required to frequently revalidate
existing, in-use entries. Failure to do so can result in the
entry being discarded. For in-use entries, discarding the entry
will almost certainly result in a subsequent request to perform
address resolution on the entry, but this time using multicast.
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As above, once the entry has been flushed, existing traffic for
destinations using that entry can no longer be forwarded until
address resolution completes successfully.
3. To maintain the stability of the control plane, Neighbor
Discovery activity related to traffic sourced by the router (as
opposed to traffic being forwarded by the router) should be given
high priority. Whenever network problems occur, debugging and
making other operational changes requires being able to query and
access the router. In addition, routing protocols dependent on
Neighbor Discovery for connectivity may begin to react
(negatively) to perceived connectivity problems, causing
additional undesirable ripple effects.
4. Traffic to unknown addresses should be given lowest priority.
Indeed, it may be useful to distinguish between "never seen"
addresses and those that have been seen before, but that do not
have a corresponding NCE. Specifically, the conceptual
processing algorithm in IPv6 Neighbor Discovery [RFC4861] calls
for deleting NCEs under certain conditions. Rather than delete
them completely, however, it might be useful to at least keep
track of the fact that an entry at one time existed, in order to
prioritize address resolution requests for such neighbors
compared with neighbors that have never been seen before.
7.2. Queue Tuning
On implementations in which requests to NDP are submitted via a
single queue, router vendors should provide operators with means to
control both the rate of link-layer address resolution requests
placed into the queue and the size of the queue. This will allow
operators to tune Neighbor Discovery for their specific environment.
The ability to set, or have per-interface or per-prefix queue limits
at a rate below that of the global queue limit might restrict the
damage to the Neighbor Discovery processing to the network targeted
by the attack.
Setting those values must be a very careful balancing act -- the
lower the rate of entry into the queue, the less load there will be
on the ND process; however, it will take the router longer to learn
legitimate destinations as a result. In a datacenter with 6,000
hosts attached to a single router, setting that value to be under
1000 would mean that resolving all of the addresses from an initial
state (or something that invalidates the address cache, such as a
Spanning Tree Protocol (STP) Topology Change Notification (TCN)) may
take over 6 seconds. Similarly, the lower the size of the queue, the
higher the likelihood of an attack being able to knock out legitimate
traffic (but less memory utilization on the router).
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